Mitigation of arc flash hazard in photovoltaic power plants

ABSTRACT

Arc flash mitigation devices are employed to protect personnel during maintenance of photovoltaic inverters. During normal operation, an alternating current (AC) output of a photovoltaic inverter is coupled to a low voltage winding of a step up transformer through a bus-bar (e.g., an electrically conductive interconnect), which has higher current rating than a fuse. During maintenance, the bus-bar is replaced with the fuse. The fuse may be employed in conjunction with a switch. The switch may be a disconnect switch that places the bus-bar in parallel with the fuse during normal operation, and decouples the bus-bar from the fuse during maintenance. The switch may also be a transfer switch that places either the bus-bar or the fuse in series with the AC output of the photovoltaic inverter and the low voltage winding of the step up transformer.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cells. More particularly, embodiments of the subject matterdisclosed herein relate to photovoltaic power plant operation andmaintenance.

BACKGROUND

Photovoltaic power plants employ solar cells to convert solar radiationto electrical energy. Photovoltaic power plants also includephotovoltaic inverters (“inverters”), which convert direct current (DC)generated by the solar cells to alternating current (AC) suitable fordelivery to a point of interconnect with a utility grid through anetwork of transformers and transmission lines. Inverters are oftenemployed in inverter stations that comprise multiple inverters connectedto a single multiple-winding medium voltage step-up transformer, whichin turn is connected to a medium voltage grid.

Arc flash is a serious workplace hazard when working on inverters, suchas during maintenance. Mitigating arc-flash hazard in inverter stationsposes several design challenges because the utility grid to which theinverter stations are connected serve as large fault current sources,leading to high arc-flash energies within the inverter stations duringarc faults. In addition, effective commissioning and maintenanceactivities often require full internal access to inverters while theinverters are powered ON and connected live to the utility grid. Thedocuments IEEE 1584-2002 by the Institute of Electrical and ElectronicEngineers and NFPA-70E by the National Fire Protection Associationprovide guidelines to analyze and estimate the arc-flash energy atvarious locations within the electrical systems such as AC inverterstations and to determine the appropriate personal protective equipment(PPE) required for protection against potential arc flash events.

For generic, i.e., not necessarily for photovoltaic applications,electric equipment, arc flash mitigation solutions may include reducingarc current, increasing the working distance, and reducing the clearingtime. These solutions, however, may be difficult to achieve orinadequate to protect workers at inverter stations.

Certain protective devices are current limiting by design. By limitingor reducing the current available for an arc fault, the correspondingincident energy is reduced during fault-clearing times that aretypically short in duration (e.g., 1-3 cycles). Fault currents at theseprotective devices must be in the current limiting range for them to beeffective. The potential problem of this solution is that below thefault current limit, the clearing time goes up significantly and,therefore, the incident energy level may exceed workable levels for arange of grid operating conditions of a photovoltaic power plant.

Increasing the working distance will significantly reduce the incidentenergy level because the incident energy is proportional to the squareof the distance in open air. Working distance can be increased by usingremote operating devices and extension tools (e.g., hot-sticks).However, in the case of inverter stations, many maintenance orcommissioning activities need to have internal access to the inverterswhile the inverters are powered ON and connected live to the utilitygrid. Therefore, increasing the working distance may not be practical ininverter stations.

One popular method of reducing clearing times is to lower the currentsetting of the protective device, such as a circuit breaker. Adisadvantage of this solution, in particular for inverter stations, isthat the level to which the current setting can be lowered is limitedfor normal operating conditions due to the need to coordinate thebreaker tripping characteristics with equipment protection needs andavoiding normal short-term transient currents from tripping the breaker.

Other solutions for mitigating arc flash hazards in general includeprovision of bus differential protection and zone selectiveinterlocking, the specific implementations of which are highlysystem-dependent, typically complex, and not cost effective.

BRIEF SUMMARY

In one embodiment, arc flash mitigation devices are employed to protectpersonnel during maintenance of photovoltaic inverters. During normaloperation, an alternating current (AC) output of a photovoltaic inverteris coupled to a low voltage winding of a step up transformer through abus-bar (e.g., an electrically conductive interconnect), which hashigher current rating than a fuse. During maintenance, the bus-bar isreplaced with the fuse. The fuse may be employed in conjunction with aswitch. The switch may be a disconnect switch that places the bus-bar inparallel with the fuse during normal operation, and decouples thebus-bar from the fuse during maintenance. The switch may also be atransfer switch that places either the bus-bar or the fuse in serieswith the AC output of the photovoltaic inverter and the low voltagewinding of the step up transformer.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures.

FIG. 1 shows a schematic diagram of a photovoltaic power plant inaccordance with an embodiment of the present invention.

FIGS. 2 and 3 show schematic diagrams of a system in the form of aninverter station in accordance with an embodiment of the presentinvention.

FIG. 4 shows a schematic diagram illustrating bus-bar links of thesystem of FIG. 2 in accordance with an embodiment of the presentinvention.

FIG. 5 shows a schematic diagram illustrating fuse links of the systemof FIG. 3 in accordance with an embodiment of the present invention.

FIG. 6 show a schematic diagram of bus-bar links on interconnect holdersin accordance with an embodiment of the present invention.

FIG. 7 show a schematic diagram of fuse links on interconnect holders inaccordance with an embodiment of the present invention.

FIG. 8 shows a schematic diagram of another system in the form of aninverter station in accordance with an embodiment of the presentinvention.

FIG. 9 schematically shows switch-fuse links of the system of FIG. 8 inaccordance with an embodiment of the present invention.

FIG. 10 schematically shows further details of the switch-fuse links ofthe system of FIG. 8 in accordance with an embodiment of the presentinvention.

FIG. 11 shows a schematic diagram of yet another system in the form ofan inverter station in accordance with an embodiment of the presentinvention.

FIG. 12 schematically shows switch-fuse links of the system of FIG. 11in accordance with an embodiment of the present invention.

FIGS. 13 and 14 schematically illustrate operation of a maintenancelever in accordance with an embodiment of the present invention.

FIG. 15 shows a flow diagram of a method of switching an inverterstation from normal operation mode to maintenance mode in accordancewith an embodiment of the present invention.

FIG. 16 shows a flow diagram of a method of switching an inverterstation from maintenance mode to normal operation mode in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus, components, and methods, to provide a thoroughunderstanding of embodiments of the invention. Persons of ordinary skillin the art will recognize, however, that the invention can be practicedwithout one or more of the specific details. In other instances,well-known details are not shown or described to avoid obscuring aspectsof the invention.

Conventional solutions to arc flash hazard described above are based onthe premise that the solution should be common to both normaloperational and maintenance conditions, i.e., the solution needs toguarantee the arc flash risk category is not higher than appropriate forsafely performing commissioning and maintenance activities whileensuring the normal operating conditions are not impacted. However, asingle setting of a protective device for both operation and maintenancedoes not guarantee the protective device can clear the fault quickly forall available short-circuit current levels. For example, a circuitbreaker may trip within 50 ms for currents higher than 1000 A but maytake over 10 s for currents less than 800 A. Given that arc flash energyis a non-linear function of available fault current and is proportionalto the fault clearing time, the highest potential short-circuit currentlevel and its corresponding clearing time may not represent theworst-case scenario in terms of arc flash energy. In fact, the highestarc flash energy may correspond to a fault current level much below themaximum.

Also, while performing arc flash hazard analysis, it is quite common toassume that the utility grid is an infinite bus or that maximum faultcurrent is available from the utility grid. However, these assumptionscould yield inaccurate results, often underestimating the arc flashenergy. In addition, these assumptions do not take into account futureplant expansions or modifications that may significantly impact theavailable fault currents. In a large photovoltaic (PV) power plant whereinverter stations are dispersed over hundreds of acres, the lineimpedances from inverter stations to the substation may varysignificantly. Therefore, the available short circuit currents and hencethe arc flash energy levels may widely vary among different inverterstations. A single point setting of the protective devices within alarge PV power plant may not be sufficient for all the aforementionedcomplexities.

Furthermore, a single point setting of tripping time for the traditionalcircuit breaker solution, when the tripping current is set low, mayintroduce nuisance tripping that may result in undesired interruptions,frequent shutdowns, and restarts that impact reliable power production.On the other hand, if the tripping current setting is too high, the arccurrents below the tripping level may still result in arc flash hazardsimply because the resulting breaker trip time increases significantly.More specifically, if the arcing current exceeds the instantaneoussetting, incident energy levels are very low. At arc currents lower thanthe instantaneous setting, the additional clearing time more thanoffsets the lower arc current to produce higher incident energy andtherefore results in a more hazardous situation.

Solutions conventionally used for generic electric equipment and systemsthat involve reducing the protection device trip setting prior toperforming maintenance activities to reduce fault clearance time andinserting a high impedance element between the utility grid and theequipment prior to performing maintenance activities to reduce faultcurrent levels are not easily extendable to inverter stations due tohigh implementation costs and the extended procedural steps necessaryfor safely performing maintenance.

Accordingly, embodiments of the present invention provide arc flashmitigation solutions that are especially advantageous, cost effective,and practical for performing maintenance on inverters.

Referring now to FIG. 1, there is shown a schematic diagram of a PVpower plant 100 in accordance with an embodiment of the presentinvention. The PV power plant 100 may include a plurality of transformerhousings 140, a plurality of photovoltaic inverters 120, a plurality ofphotovoltaic modules 110, and a high voltage (HV) step up transformer160. Control and other components of the PV power plant 100 notnecessary for the understanding of the invention are not shown forclarity of illustration.

In the example of FIG. 1, an inverter station 190 includes one or moreinverters 120 connected to a transformer housing 140. A transformerhousing 140 may include a single multiple-winding medium voltage (MV)transformer to which all inverters 120 in the same inverter station 190are connected. Groups of solar cells 115 may be packaged together in aphotovoltaic module 110, which may be connected to an inverter 120 alongwith other photovoltaic modules 110. The solar cells 115 may comprisecommercially-available solar cells, such as those available fromSunPower Corporation of San Jose Calif. It is to be noted that only someof the solar cells 115 are labeled in FIG. 1 in the interest of clarity.

An inverter 120 converts DC current generated by a set of photovoltaicmodules to AC current suitable for delivery to the utility grid at apoint of interconnect (POI) 161. In the example of FIG. 1, the output ofan inverter 120 is stepped up by an MV transformer of a transformerhousing 140 and further stepped up by the transformer 160 before beingprovided to the utility grid, which may be operated by a utility companythat provides electrical service to consumers.

FIGS. 2 and 3 show schematic diagrams of a system in the form of aninverter station 190A in accordance with an embodiment of the presentinvention. The inverter station 190A is a particular embodiment of aninverter station 190 of the PV power plant 100 shown in FIG. 1. Theinverter station 190A is illustrated using two inverters 120 (i.e.,120-1, 120-2) that are connected to a single multiple winding step up MVtransformer 142. In general, an inverter station may have fewer or moreinverters connected to an MV transformer. In one embodiment, the MVtransformer 142 has two low voltage windings and a single high voltagewinding. Each of the low voltage windings is coupled to an inverter 120and the high voltage winding is coupled to the utility grid.

The transformer housing 140A shown in FIG. 2 is a particular embodimentof a transformer housing 140 of the PV power plant 100. The transformerhousing 140A may enclose the MV transformer 142, an MV expulsion fuse143, an MV current limiting fuse 144, and an MV disconnect switch 145 inthe same protective housing for safety reasons. As will be more apparentbelow, a transformer housing may include different inverter-transformerlinks (e.g., bus-bar or contact links, fuse links, and switch-fuselinks) within its housing in various embodiments of the presentinvention. The inverter-transformer links, which connect the inverters120 to the MV transformer 142, may also be located outside a transformerhousing.

The inverter station 190A is an example of a configuration with twoinverters 120 connected to a single three-winding MV transformer 142.Other circuit configurations of an inverter station may include a singleinverter or multiple parallel inverters connected through a two windingMV transformer.

An inverter 120 may comprise an inverter circuit 122, an AC outputdisconnect 123 and a DC input disconnect 121. The inverter circuit 122comprises an electrical circuit for converting the DC power receivedfrom the photovoltaic modules 110 to a utility grid compatible output.The DC input disconnect 121 and the AC output disconnect 123 maycomprise switches for disconnecting the inverter 120 from thephotovoltaic modules 110 and the MV transformer 142, respectively. Theinverters 120 may comprise commercially available photovoltaicinverters.

An inverter 120 has a DC input for receiving the DC output of thephotovoltaic modules 110 and an AC output that is provided to theutility grid. The AC output of the inverter 120 is stepped up by the MVtransformer 142 before being provided to the utility grid. In additionto stepping up the AC voltage, the MV transformer 142 provides galvanicisolation between the utility grid and the inverters 120, and hence tothe photovoltaic modules 110.

Given that the inverters 120 are connected to a large utility grid withpotentially large available fault current, potential for the occurrenceof arc flash at the inverters 120 is a major safety concern. Althoughovercurrent protection is typically provided at the high voltage side ofthe MV transformer 142 by the fuses 143 and 144, the corresponding faultclearance times for faults at the low voltage side of the MV transformer142 and the inverters 120 are generally long and widely varies,resulting in high incident arc energies in the region between the lowvoltage terminals of the MV transformer 142 and the AC outputs of theinverters 120. Insertion of an appropriate arc flash mitigation solutionat this location between the MV transformer 142 and the inverters 120will help ensure personnel safety, specifically while operating,commissioning, maintaining, or servicing the inverters 120. Preferably,the arc flash mitigation solution reduces the incident energy level fromPPE class 4 or higher (>40 cal/cm²) to PPE class 2 or lower (<8 cal/cm²)consistently for all potentially available fault currents from theutility grid.

A conventional arc flash mitigation solution is to install dedicatedcircuit breakers within the MV transformer housing and in series withthe inverter outputs. The circuit breakers help ensure that anypotential occurrence of arc fault at the inverter outputs is instantlycleared by the associated circuit breaker such that the associatedincident energies do not exceed PPE class 2 level. However, propersetting of circuit breaker tripping characteristics is a difficult taskat best because the setting needs to ensure that the circuit breakertrips instantly for a wide range of arc fault currents with levelsdependent on the available fault currents, while not responding to overand surge currents expected during normal operation. Also, in the caseswhere the circuit breakers are located within the MV transformerhousing, the housing should be suitably designed to meet theenvironmental requirements of the circuit breakers, resulting inincreased implementation costs.

In one embodiment, the inverter station 190A may have differentinverter-transformer links depending on whether the inverter station190A is on normal operation mode or maintenance mode. As its nameimplies, normal operation mode is when the inverters 120 are normallyoperating to provide solar generated power to the utility grid.Maintenance mode is when the inverters 120 are being maintained,serviced, or commissioned.

FIG. 2 shows the connection configuration of the outputs of theinverters 120 to the MV transformer 142 in normal operation mode. Innormal operation, a bus-bar link 141 (e.g., an electrically conductiveinterconnect) serves as an inverter-transformer link connecting aninverter 120 to the MV transformer 142. A bus-bar link 141 ispreferably, but not necessarily, enclosed within the transformer housing140A. A single wire or multi-wire conductor 124 may connect a bus-barlink 141 to an AC output of an inverter 120.

During normal operation, the bus-bar links 141 connect the AC outputs ofthe inverters 120 to corresponding low voltage terminals of the MVtransformer 142. The bus-bar links 141 are designed such that they carrythe full rated current of the inverters 120, and are arranged in amechanical configuration such that they can be easily removed andreplaced with a set of fuse-links 146 (see FIG. 3) for maintenance mode.FIG. 4 shows a schematic diagram illustrating bus-bar links 141connecting the terminals L1A, L2A, and L3A from the inverter 120-1 tocorresponding low voltage winding terminals X1A, X2A, and X3A of the MVtransformer 142, and connecting the terminals L1B, L2B, and L3B from theinverter 120-2 to corresponding low voltage winding terminals X1B, X2B,and X3B of the MV transformer 142. The example shown is for athree-phase wiring, one wiring for each phase, for illustration purposesonly.

FIG. 3 shows the connection configuration of the inverters 120 to the MVtransformer 142 in maintenance mode. In maintenance mode, fuse links 146(instead of the bus-bar links 141) serve as the inverter-transformerlinks. Prior to performing a maintenance task on the inverters 120, eachbus-bar link 141 is replaced with a fuse link 146. Unlike the bus-barlinks 141, which are rated to carry the full rated current of theinverters 120, the fuse-links 146 are rated to carry a fraction of therated current of the inverters 120 such that fault-clearance time of thefuse-links 146 is sufficiently short to reduce arc flash energy wellbelow the levels corresponding to PPE class 2 for the full range ofavailable fault currents. A fuse-link 146 may comprise a fast actingfuse. With the fuse links 146, maintenance tasks that require theinverters 120 to remain powered ON may thus be employed in relativesafety. Examples of maintenance tasks that require the inverters 120 tobe powered ON include inverter output current and voltage measurements,leakage current measurements, harmonics measurements, thermalmeasurements, control, communication and monitoring circuits andfunctionalities diagnostics, or any other multimeter or oscilloscopemeasurements that may arise during maintenance and commissioningactivities that require close access to live components inside theinverters. Should an arc fault situation were to arise duringmaintenance, the fast acting fuse links 146 are able to clear the arcwithin a few milliseconds to a few hundred milliseconds, therebylimiting the incident energy significantly well below 8 cal/cm². Thisallows maintenance personnel to safely perform maintenance tasks on theinverter station 190 using only PPE 2 level protection. On completion ofthe maintenance task, the fuse links 146 are removed and replaced withthe bus-bar links 141 so that normal operation can be resumed. FIG. 5shows a schematic diagram illustrating fuse links 146 connecting theterminals L1A, L2A, and L3A from the inverter 120-1 to corresponding lowvoltage winding terminals X1A, X2A, and X3A of the MV transformer 142,and connecting the terminals L1B, L2B, and L3B from the inverter 120-2to corresponding low voltage winding terminals X1B, X2B, and X3B of theMV transformer 142.

In one embodiment, the bus-bar links and the fuse links share a commonphysical spacing layout, termination footprint, and termination devices,such that procedures for replacing bus-bar links 141 with fuse links146, and vice versa, can be performed reliably with ease and withminimal number of steps. This feature is schematically illustrated inFIGS. 6 and 7 where an interconnect holder 192 physically accommodateseither a bus-bar link 141 or a fuse link 146. Each holder 192 allows forconnecting a low voltage winding terminal of the MV transformer 142(i.e., X1A, X2A, X3A, X1B, X2B, or X3B) to a corresponding terminal froman inverter 120 (i.e., L1A, L2A, L3A, L1B, L2B, or L3B). To change fromnormal mode to maintenance mode, the maintenance person simply has toremove the bus-bar links 141 from the interconnect holders 192 andinstall the fuse links 141 onto the holders 192. Similarly, to changefrom maintenance mode to normal operation mode, the maintenance personsimply has to remove the fuse links 146 from the interconnect holders192 and install the bus-bar links 141 onto the holders 192. During theswapping of the bus-bar links 141 with the fuse links 146 and viceversa, the entire inverter station 190A needs to be powered OFF anddisconnected from the utility grid, which may be achieved by opening theMV disconnect switch 145 (shown in FIGS. 2 and 3), which is typicallypresent at the high voltage side of the MV transformer 142.

With reference to FIGS. 2 and 3, a method of performing maintenance onan inverter 120 of a PV power plant 100 in one embodiment of theinvention may involve putting the inverter station 190A in maintenancemode by powering OFF the inverters 120, disconnecting the inverterstation 190A from the utility grid by opening the MV disconnect switch145, replacing the bus-bar links 141 with the fuse-links 146, closingthe MV disconnect switch 145, powering ON the inverters 120, andperforming maintenance on one or more inverters 120 (while powered ON)with the fuse-links 146 in-place instead of the bus-bar links 141. Themethod further involves putting the inverter station 190A in normaloperation mode by powering OFF the inverters 120, disconnecting theinverter station 190A from the utility grid by opening the MV disconnectswitch 145, replacing the fuse links 146 with the bus-bar links 141,closing the MV disconnect switch 145, and powering ON the inverters 120with the bus-bar links 141 in place instead of the fuse links 146.

Referring now to FIG. 8, there is shown a schematic diagram of a systemin the form of an inverter station 190B in accordance with an embodimentof the present invention. The inverter station 190B is a particularembodiment of the inverter station 190A where the inverter-transformerlinks comprise switch-fuse links 201. The switch-fuse links 201 may beincorporated within the protective enclosure of the transformer housing,which is relabeled as “140B.” Other components of the inverter 190B areotherwise as described with reference to FIGS. 2 and 3.

In one embodiment, as shown in FIG. 8, a switch-fuse link 201 comprisesa fast acting fuse F and a single-throw switch 203. Like a fuse-link146, the fuse F is rated for a fraction of the rated current of theinverters 120 such that fault-clearance time of the fuse F issufficiently short to reduce the arc flash energy well below the levelscorresponding to PPE2. The single-throw switch 203 may comprise atwo-terminal single-throw disconnect switch.

During normal operation, the switch 203 is closed and in parallel withthe fuse 201. The contacts B of switch 203 thus connect the inverter 120to the MV transformer 142 during normal operation, just like thepreviously described bus-bar link 141 (see FIG. 2). Because of theparallel connection of the contact B of the switch 203 and the fuse F,the fuse F carries only a small fraction of the rated current of theinverter 120 during normal operation. During maintenance mode, theswitch 203 is opened from being in parallel with the fuse F, allowingthe fuse F to connect the inverter 120 to the MV transformer 142 justlike the previously described fuse-links 146 (see FIG. 3).

FIG. 9 schematically shows the switch-fuse links 201 in accordance withan embodiment of the present invention. As explained, a switch-fuse link201 may comprise a fuse F (i.e., F1, F2, F3, F4, F5, or F6) and acorresponding switch contacts B (i.e., B1, B2, B3, B4, B5, or B6) ofswitch 203. In the example of FIG. 9, the single-throw switches 203 areganged together to be operable by a single maintenance lever 204, whichmay be accessible on the outside of the transformer housing. The MVtransformer 142 winding terminals X1A, X2A, X3A, and X1B, X2B, X3B andthe terminals L1A, L2A, L3A, L1B, L2B, and L3B from the inverters 120are as previously described with reference to FIGS. 4 and 5.

In the example of FIG. 9, throwing the maintenance lever 204 into afirst position (normal operation position) closes all the contacts B ofthe switches 203 to place the contacts B in parallel with correspondingpermanently installed fuses F to place the inverter station 190B innormal operation mode. To place the inverter station 190B in maintenancemode, the maintenance lever 204 is thrown into a second position(maintenance position) to open the contacts B of the switches 203 suchthat the inverters and the MV transformer low voltage windings areconnected through the fuses F. The maintenance lever 204 may be lockedin place in a particular position with a padlock 205 for safety reasons.The switch-fuse links 201 advantageously eliminate the need to manuallyremove and install bus-bars and fuses to change mode of operations.

In the switch-fuse links 201, because the fuses F are always installed,the fuses F are still present and connected in parallel with thecontacts B during normal operation. Consequently, each fuse F will carrya small fraction of the normal operating current. The amount of fusecurrent flowing during the normal operating condition may be reduced toa very small level by a circuit layout design of a switch-fuse link 201with appropriate impedance matching between the contact B of a switch203 and the fuse F. This is illustrated in FIG. 10, which schematicallyshows a switch-fuse link 201 in accordance with an embodiment of thepresent invention.

In the example of FIG. 10, a circuit 206 that comprises the contact B ofswitch 203 has an equivalent series resistance 207. A circuit 208 thatcomprises the fuse F has an equivalent series resistance 209. If, forexample, the equivalent series resistance 207 of the circuit 206associated with the switch 203 is R_(swi), then the circuit 208 of thecorresponding fuse F is intentionally designed such that the equivalentseries resistance 209 is equal to k_(i)R_(swi), where k_(i)>>1. Theequivalent impedances should be designed to meet environmentalrequirements for both normal operating and maintenance conditions.

Turning now to FIG. 11, there is shown a schematic diagram of a systemin the form of an inverter station 190C in accordance with an embodimentof the present invention. The inverter station 190C is a particularembodiment of the inverter station 190A where the inverter-transformerlinks comprise switch-fuse links 212. The switch-fuse links 212 may beincorporated within the protective enclosure of the transformer housing,which is relabeled as “140C.” Other components of the inverter 190C areotherwise as described with reference to FIGS. 2 and 3.

In one embodiment depicted in FIG. 11, a switch-fuse link 212 comprisesa fast acting fuse 214, a bus-bar 213, and a double-throw switch 215.Like a fuse-link 146, the fuse 214 is rated for a fraction of the ratedcurrent of the inverters 120 such that fault-clearance time of the fuse214 is sufficiently short to reduce the arc flash energy well below PPE2. The double-throw switch 215 may comprise a transfer switch with twopositions to connect the inverter 120 to the MV transformer 142 eitherthrough the fuse 214 or the bus-bar 213. That is, only one of the fuse214 or the bus-bar 213 is connecting the inverter 120 to the low-voltageside of the MV transformer 142 at any given time. The fuse 214 and thebus-bar 213 may be permanently installed.

An advantage of using switch-fuse links 212 instead of switch-fuse links201 (see FIG. 8) is that the fuses 214 do not carry any load currentduring normal operation. Consequently, the fuse sizing and circuitlayout design considerations do not need to take into account continuousduty requirements, resulting in simpler electrical design. A trade-offis that the switch-fuse links 212 are typically more expensive toimplement and physically bigger than the switch-fuse links 201.

FIG. 12 schematically shows the switch-fuse links 212 in accordance withan embodiment of the present invention. In the example of FIG. 12, thedouble-throw switches 215 are ganged together to be operable by thesingle maintenance lever 204, which may be accessible on the outside ofthe transformer housing. The MV transformer 142 winding terminals X1A,X2A, X3A, and X1B, X2B, X3B and the terminals L1A, L2A, L3A, L1B, L2B,and L3B from the inverters 120 are as previously described withreference to FIGS. 4 and 5.

In the example of FIG. 12, throwing the maintenance lever 204 into afirst position connects the inverters 120 to the MV transformer 142through series-connected bus-bar 213 to place the inverter station 190Cin normal operation mode. To place the inverter station 190C inmaintenance mode, the maintenance lever 204 is thrown into a secondposition to connect the inverters 120 to the MV transformer 142 throughseries-connected fuse 214. The maintenance lever 204 may be locked inplace in a particular position with a padlock 205 for safety reasons.

In both the inverter stations 190B (see FIG. 8) and 190C (see FIG. 11),the procedure for switchover between normal operation and maintenancemodes is performed with the inverter station completely powered OFF anddisconnected from the utility grid to ensure that the switchingprocedure itself does not pose an arc flash hazard. This may beaccomplished by opening the MV disconnect switch 145 (typically througha hot-stick) prior to operating the maintenance lever 204. An interlockmechanism may be employed to ensure the switchover between the two modesof operation is accomplished only with the MV disconnect switch 145 inthe open (i.e., OFF) position.

FIGS. 13 and 14 show the maintenance lever 204 in normal operationposition (FIG. 13) and maintenance position (FIG. 14). The maintenancelever 204 may be locked in place using the padlock 205. When themaintenance lever 204 is placed in the normal operation position toplace the inverter station in normal operation mode, the inverters 120are connected to the low-voltage side of the MV transformer 142 throughseries connected switch contacts and/or bus-bars, e.g., contact B ofswitch 203 in the case of the switch-fuse links 201 and bus-bars 213 andthe contact of switch 215 in the case of the switch-fuse links 212. Whenthe maintenance lever 204 is placed in the maintenance position to placethe inverter station in maintenance mode, the inverters 120 areconnected to the low-voltage side of the MV transformer 142 throughseries connected switch contacts and/or fuses, e.g., fuses F in the caseof the switch-fuse links 201 and fuses 214 and contact of switch 215 inthe case of the switch-fuse links 212.

FIG. 15 shows a flow diagram of a method of switching an inverterstation 190 from normal operation mode to maintenance mode in accordancewith an embodiment of the present invention. The method of FIG. 15 maybe performed as a pre-maintenance procedure.

In the method of FIG. 15, the inverters 120 are powered OFF, anddisconnected by opening their DC input disconnects 121 and AC outputdisconnects 123 (step 301). The MV disconnect switch 145 is opened(i.e., switched OFF) and locked in the open position with a loaddisconnect padlock having the same padlock key as the padlock 205 of themaintenance lever 204 (step 302); the padlock key is then released fromthe load disconnect padlock. The padlock key is used to unlock themaintenance lever 204, and the maintenance lever 204 is turned from theoperation position to the maintenance position (step 303). Themaintenance lever 204 is locked at the maintenance position and thepadlock key is released (step 304). The doors of the inverter 120 areopened to perform any power-off maintenance actions, and then closedafter the maintenance actions have been performed (step 305). Step 305is repeated for all the inverters 120 in the inverter station (step306). After performing the power-off maintenance actions, the loaddisconnect padlock is unlocked using the padlock key to allow the MVdisconnect switch 145 to be closed, i.e., switched ON (step 307). Theinverters 120 are powered ON, and reconnected by closing their DC inputdisconnects 121 and AC output disconnects 123 (step 308). Prior topowering on, the output power limit of the inverters 120 are set to avalue smaller than the full rating that corresponds with, and isappropriate for, the rating of the fuses F of the switch-fuse link 201or fuses 214 of the switch-fuse link 212. Power-on maintenance actionsare then performed on the inverters 120. Note that during the power-onmaintenance actions (i.e., maintenance actions performed while theinverters 120 are powered ON), the output power level of the inverters120 are still limited as determined by the fuse ratings of theswitch-fuse links 201 or 212, whichever is implemented, because themaintenance lever 204 is still locked in the maintenance position.

FIG. 16 shows a flow diagram of a method of switching an inverterstation 190 from maintenance mode to normal operation mode in accordancewith an embodiment of the present invention. The method of FIG. 16 maybe performed as a post-maintenance procedure following the method ofFIG. 15.

In the method of FIG. 16, the inverters 120 are powered OFF, anddisconnected by opening their DC input disconnects 121 and AC outputdisconnects 123 (step 320). The MV disconnect switch 145 is opened(i.e., switched OFF) and locked in the open position with the loaddisconnect padlock (step 321); the padlock key is then released from theload disconnect padlock. The padlock key is used to unlock themaintenance lever 204, and the maintenance lever 204 is turned from themaintenance position to the operation position (step 322). Themaintenance lever 204 is locked in the operation position and thepadlock key is released. The disconnect padlock of the MV disconnectswitch 145 is unlocked, allowing the MV disconnect switch 145 to beclosed (step 323). The MV disconnect switch 145 is locked in the closedposition and the padlock key 205 is left captive in the disconnectpadlock. The inverters 120 are powered ON, and reconnected by closingtheir DC input disconnects 121 and AC output disconnects 123 (step 324).The output power limit of the inverters 120 are then adjusted back totheir full rating

Note that in embodiments of the invention, the fuses limit the amount ofpower that can be transferred from the inverters to the utility grid dueto their lower current rating in relation to the full rating of theinverters. This limitation does not impact typical maintenanceactivities and associated tests on the inverters as they do not requirethe need for full power operation during maintenance. Due to thislimitation, during maintenance, as described in step 308 of FIG. 15, thepower production from the inverters needs to be curtailed to within thelimits allowed by the fuse rating, and this typically can beaccomplished by software control of the inverters or by disconnection ofseveral DC input circuits feeding into the inverters.

Table 1 shows a qualitative comparison of embodiments of the inventionthat employ switch-fuse links against a conventional solution that isbased on circuit breakers. Although the material costs associated withthe switch-fuse based solutions of FIGS. 8 and 11 are higher than thatof the bus-bar and fuse link based solution of FIGS. 2 and 3, due to thevoltage and high current rating of the switches and use of permanentlyinstalled fuses, the switch-fuse based solutions are still costeffective when compared to circuit breaker-based solutions. Moreimportantly, both switch-fuse based solutions provide assured safetyagainst potential arc-flash hazards in inverter stations, while thecircuit breaker-based solution does not guarantee such assurance inparticular when the available fault current and the resulting arc-flashcurrent vary and can reach to a level that is lower than the fixed fasttripping current level of the circuit breaker. Also, depending on thesize and the specific maintenance needs of the PV power plant, thebus-bar and fuse link based solution of FIGS. 2 and 3 may be the mostcost effective solution.

TABLE 1 Fuse + Double- Fuse + Single- Bus-bar + Fuse throw Switch throwSwitch Link Solution Circuit Breaker Solution Solution (FIGS. 2 andSolution (FIG. 11) (FIG. 8) 3) Estimated relative 1 0.6 0.5 0.3 cost ofthe solution (per unit) Power available Full Limited by the fuse sizing.Will require power during maintenance/ curtailment of the inverterscommissioning Electrical design Medium Medium High Low complexityPhysical design Medium High High Medium complexity Environmental HighMedium-High Medium-High Low design complexity Impact on normal Potentialfor None operation nuisance tripping Arc-flash May not be Assuredmitigation during assured for maintenance situations with varied and lowavailable fault currents Impact on None Medium Medium High maintenanceoperational steps PPE level during Dependent on Solution can be tailoredto meet PPE 2 or less maintenance the variation of the fault currentavailability

While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. Many additional embodiments will be apparentto persons of ordinary skill in the art reading this disclosure.

What is claimed is:
 1. A system comprising: a first photovoltaicinverter that converts direct current (DC) generated by solar cells toalternating current (AC) that is provided at an AC output of the firstphotovoltaic inverter; a step up transformer having a first low voltagewinding on a low voltage side and a high voltage winding on a highvoltage side, the low voltage winding being coupled to the photovoltaicinverter, and the high voltage winding being coupled to a utility grid;a first switch-fuse link comprising a first fuse and a first switch, thefirst switch-fuse link coupling the AC output of the first photovoltaicinverter to the first low voltage winding of the step up transformerthrough a contact of the first switch during a normal operation mode,the first switch-fuse link coupling the AC output of the firstphotovoltaic inverter to the first low voltage winding of the step uptransformer through the first fuse during a maintenance mode; a secondphotovoltaic inverter that converts DC generated by solar cells to ACthat is provided at an AC output of the second photovoltaic inverter;and a second switch-fuse link comprising a second fuse and a secondswitch, the second switch-fuse link coupling the AC output of the secondphotovoltaic inverter to a second low voltage winding of the step uptransformer through a contact of the second switch during the normaloperation mode, the second switch-fuse link coupling the AC output ofthe second photovoltaic inverter to the second low voltage winding ofthe step up transformer through the second fuse during the maintenancemode.
 2. The system of claim 1 wherein the first switch comprises asingle-throw disconnect switch.
 3. The system of claim 1 wherein thefirst fuse is coupled to the AC output of the first photovoltaicinverter and to the first low voltage winding of the step up transformerin both the normal operation mode and the maintenance mode.
 4. Thesystem of claim 1 wherein the first switch comprises a double-throwtransfer switch.
 5. The system of claim 1 wherein the first switch-fuselink is installed within a protective enclosure that houses the step uptransformer.
 6. The system of claim 1 wherein the first switch has afirst position that places the contact of the first switch in parallelwith the first fuse during the normal operation mode and a secondposition that removes the contact of the first switch from being inparallel with the first fuse during the maintenance mode.
 7. The systemof claim 1 wherein the first and second switches are ganged together toa lever, the lever having a first position that opens the first andsecond switches during the maintenance mode and a second position thatcloses the first and second switches during the normal operation mode.8. The system of claim 1 wherein the first and second switches areganged together to a lever, the lever having a first position thatcouples the contact of the first switch to the AC output of firstphotovoltaic inverter and to the first low voltage winding of the stepup transformer and that couples the contact of the second switch to theAC output of the second photovoltaic inverter and to the second lowvoltage winding of the step up transformer during the normal operationmode, and a second position that couples the first fuse to the AC outputof the first photovoltaic inverter and to the first low voltage windingof the step up transformer and that couples the second fuse to the ACoutput of the second photovoltaic inverter and to the second low voltagewinding of the step up transformer during the maintenance mode.
 9. Asystem comprising: a first photovoltaic inverter that converts directcurrent (DC) generated by solar cells to alternating current (AC) thatis provided at an AC output of the first photovoltaic inverter; a stepup transformer having a first low voltage winding on a low voltage sideand a high voltage winding on a high voltage side, the high voltagewinding being coupled to a utility grid; a first fuse; and a firsttransfer switch having a first position and a second position, the firstposition of the first transfer switch coupling a first bus-bar throughthe transfer switch contact but not the first fuse to the AC output ofthe first photovoltaic inverter and to the first low voltage winding ofthe step up transformer, the second position of the first transferswitch coupling the first fuse through the transfer switch contact butnot the first bus-bar to the AC output of the first photovoltaicinverter and to the first low voltage winding of the step uptransformer.
 10. The system of claim 9 wherein the first switchcomprises a double-throw transfer switch.
 11. The system of claim 9wherein the first fuse and the bus-bar are installed within a protectiveenclosure that houses the step up transformer.
 12. The system of claim 9further comprising: a second photovoltaic inverter that converts DCgenerated by solar cells to AC that is provided at an AC output of thesecond photovoltaic inverter; a second fuse; and a second transferswitch having a first position and a second position, the first positionof the second transfer switch coupling a second bus-bar but not thesecond fuse to the AC output of the second photovoltaic inverter and toa second low voltage winding of the step up transformer, the secondposition of the second transfer switch coupling the second fuse but notthe second bus-bar to the AC output of the second photovoltaic inverterand to the second low voltage winding of the step up transformer. 13.The system of claim 12 wherein the first and second transfer switchesare ganged together to a lever, the lever having a first position thatcouples the first bus-bar to the AC output of first photovoltaicinverter and to the first low voltage winding of the step up transformerand that couples the second bus-bar to the AC output of the secondphotovoltaic inverter and to the second low voltage winding of the stepup transformer during a normal operation mode, and a second positionthat couples the first fuse to the AC output of first photovoltaicinverter and to the first low voltage winding of the step up transformerand that couples the second fuse to the AC output of the secondphotovoltaic inverter and to the second low voltage winding of the stepup transformer during a maintenance mode.
 14. The system of claim 13wherein the first transfer switch, the second transfer switch, the firstbus-bar, and the second bus-bar are located within a protectiveenclosure that houses the step up transformer.
 15. The system of claim14 wherein the lever is located outside the protective enclosure thathouses the step up transformer.
 16. A method of performing maintenanceon a photovoltaic inverter comprising: powering OFF the photovoltaicinverter; replacing a bus-bar that couples an alternating current (AC)output of the photovoltaic inverter to a low voltage winding of a stepup transformer with a fuse; powering ON the photovoltaic inverter; andperforming maintenance on the photovoltaic inverter while thephotovoltaic inverter is powered ON and the fuse is in place instead ofthe bus-bar.
 17. The method of claim 16 further comprising: powering OFFthe photovoltaic inverter after performing maintenance on thephotovoltaic inverter; replacing the fuse with the bus-bar; and poweringON the photovoltaic inverter for normal operation with the bus-bar inplace instead of the fuse.
 18. The method of claim 16 furthercomprising: disconnecting a high voltage side of the step up transformerprior to replacing the bus-bar with the fuse.
 19. The method of claim 16wherein the method is performed for another photovoltaic inverter in asame inverter station as the photovoltaic inverter.